America's National Security Agency has been accused of spying on the online sexual activity of Islamist radicals in order to find ways to discredit them.

The Huffington Post has published a document, leaked by ex-NSA contractor Edward Snowden, which claimed the organisation tracked six unnamed "radicalisers" and their visits to pornographic websites.

It said the alleged electronic surveillance aimed to find their "personal vulnerabilities" to undermine their credibility.

The website published parts of the document, which is dated October 3, 2012, but redacted the names of the six people.

It said none of the six were accused of being involved in terror plots. It added that the NSA believed all those targeted lived outside the US.

"Some of the vulnerabilities, if exposed, would likely call into question a radicaliser's devotion to the jihadist cause, leading to the degradation or loss of his authority," the document read.

When contacted for comment, the NSA referred The Associated Press to a statement from the Office of the Director of National Intelligence.

"Without discussing specific individuals, it should not be surprising that the US Government uses all of the lawful tools at our disposal to impede the efforts of valid terrorist targets who seek to harm the nation and radicalize others to violence," the statement said.